1. Field of the Invention
The present invention relates generally to cardiac stimulator and arrhythmia therapeutic devices, and more specifically to an anti-arrhythmia device including multiple stimulators. Still more particularly, the invention relates to a medical device including a planet electronics assembly and an array of implantable satellite stimulators controlled by and receiving power, partly or wholly, from the planet via wireless communications links,
2. Background of the Invention
The human heart pumps blood to the lungs to absorb oxygen and then pumps the oxygenated blood to the tissues of the body. After the oxygen is removed from the blood by the tissues, the oxygen-depleted blood returns to the heart and the process repeats itself. The heart comprises four chambers-two atria and two ventricles. Once the atria fill with the blood, the atria contract forcing blood into the ventricles. After the ventricles fill with blood, the ventricles, in turn, contract forcing blood to the lungs (from the right ventricle) and to the rest of the body (from the left ventricle).
The chambers of the heart contract in response to electrical signals or "wavefronts." In the normal human heart, a collection of cardiac cells referred to as the "sinus" node (or "sinoatrial node") constitutes the primary natural pacemaker by which rhythmic electrical excitation is developed to cause the chambers to contract. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers (or atria) at the right and left sides of the heart. The impulse from the sinus node is transmitted to the ventricles through the atrioventricular node. The transmitted impulse causes the ventricles to contract. The cycle of events during which an electrical impulse is conducted through the heart causing contraction of the atria followed by contraction of the ventricles is referred to as a "cardiac cycle."
Disruption of the heart's natural pacemaking system as a result of aging, disease or surgical intervention is commonly treated by artificial cardiac pacing, by which rhythmic electrical discharges are applied to the heart at a desired rate from an artificial pacemaker. An artificial pacemaker (or "pacer" as it is commonly labeled) is a medical device which typically senses electrical impulses and delivers electrical pulses to one or more electrodes that are implanted adjacent to or in the patient's heart in order to stimulate the heart to contract at a desired rate. If the body's natural pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, an implantable pacemaker often is required to properly stimulate the heart. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Pat. No. 4,830,006.
Conventional pacers thus include an electronics assembly housed in a hermetically sealed enclosure, and one or more leads which connect the pacer directly to the heart tissues to be stimulated and sensed. By using a lead, which may be, for example, 18-30 inches in length, the electronics assembly can be implanted in a suitable area of the body, commonly the upper thorax. One end of the lead connects to the pacer, while the other end of the lead, referred to as the "distal" end, is attached to an interior surface of one of the chambers of the heart. One or more electrodes typically are disposed at the distal end of the lead through which electrical pulses are delivered to the heart at the site of the electrodes and/or from which sensing occurs. During implantation of conventional pacer systems, it is a common procedure for the physician to insert a stiff wire ("stylette") through the center of the lead and then to "snake" the lead through a predetermined path to the heart. Often the leads are implanted by guiding them through blood vessels into one or more chambers of the heart. The leads typically pass through valves that separate the atrial from the ventricular chambers.
In addition to supplying stimulating pulses to the heart, an important function performed by most modern pacemakers is sensing the electrical activity of the heart. The term "sensing" means to monitor the intrinsic electrical activity of the heart which normally precedes cardiac contraction. Conventional pacers thus include monitoring (or "sense") circuitry, such as amplifiers and filters, to process the electrical signals detected by the electrodes implanted in the heart. The processed signals are then compared to preset threshold signals used to determine whether cardiac function is within acceptable bounds. If the sense circuitry determines that the patient's heart is not beating in an acceptable manner, the pacer may provide an electrical pulse or typically, a sequence of pulses, through the lead and electrodes to artificially stimulate the heart to beat in a predetermined manner.
Although leads have been used for many years in conjunction with implanted pacemakers and defibrillators both to stimulate the heart to beat as well as to sense the electrical activity of the heart, the use of leads is not problem free. For instance, the implantation avenues available for leads to be routed to and through the heart may be limited by the lumenal diameter of the vessels leading to the heart or by valves in the heart. Chronic fixation of the lead may be influenced by anchorage available (e.g. trabeculas). Further, introduction of leads into the right side of the heart is usually preferred for implantation of pacer leads because of the reduced risk of blood loss, as the pressures in the right cardiac chambers are markedly lower than the pressures in the left cardiac chambers. Thus, for these practical reasons, a physician typically only implants the leads in a relatively few preselected sites in the heart. These sites, however, are not necessarily the optimal sites for sensing the electrical activity of the heart, but are chosen as a compromise between the complications described above and the patient's cardiac problem. Rather than monitoring the electrical activity in the right ventricle, monitoring the left ventricle's electrical activity, for example, might be preferred instead.
Additionally, it may desired to sense electrical activity at three, four, or more sites in the heart. Some pacers may be implanted with four leads permitting sensing at four different sites in the heart. Four leads may be difficult to implant as they occupy a relatively large volume in the blood vessels through which they are passed and sometimes have to be steered along circuitous routes. Further, it is becoming increasingly desirable to sense at more locations in or on the heart than is possible with conventional pacer-lead combinations. It would thus be highly beneficial to have a stimulation and sensing system that provides the diagnostic and therapeutic functions provided by conventional cardiac stimulators yet which employs fewer interconnecting leads, than required by conventional devices, or which can function without using any leads.
Two broad categories of arrhythmias include "bradycardia," which is characterized by a relatively slow heart rhythm, and "tachycardia," which is characterized by a relatively fast rhythm. It is generally known that slow "depolarization wavefront" propagation across the heart such as that caused by conduction block, gives rise to bradycardic conditions. The term "depolarization wavefront" refers to the spatial distribution of electrical charge across the heart as the heart contracts. The pacer "senses" the "depolarization wavefront" as it passes the site of electrodes. Throughout this disclosure "depolarization wavefronts" (or simply depolarization waves) is used synonymously with "sense events."
It is also generally known that tachycardic conditions arise from circus motions which are "depolarization wavefronts" which move around localized regions of cardiac tissue, such as that described in a book by W. A. Tacker and L. A. Geddes entitled "Electrical Defibrillation," CRC Press, 1980. Several circus motions may occur simultaneously, giving rise to chaotic, rotor motion characteristic of life-threatening fibrillation. Present means to control or inhibit these conditions generally require delivering enough energy via the lead electrodes to "capture" a critical volume of repolarizable tissue all at once. In so doing, a major portion of the cardiac tissue of the affected heart chamber is induced into a non-polarizable, refractory, state from which it can recover by means of artificial or natural pacing. Because brady and tachycardia often are arrhythmias occurring in a localized area of the heart, some conventional pacers and defibrillators are unable to effectively and consistently detect the onset of these conditions because of the limited number of leads and the compromise in lead locations. It would thus be desirable to be able to sense the heart's electrical activity at numerous (i.e., two or more and preferably four or more) locations within the heart.
For the foregoing reasons, a cardiac stimulator that reduces or eliminates the problems associated with conventional pacers that require leads is needed. Such a stimulator would reduce the number of leads, or eliminate the use of leads altogether.